1. Field of the Invention
The present invention relates to a fluorescence observing apparatus for measuring fluorescence emitted from a sample (e.g., an organism, etc.) by irradiation of excitation light to provide information which is used for diagnosis, etc.
2. Description of the Related Art
A diagnosis instrument, etc., for acquiring the intensity and spectrum of fluorescence emitted from a sample (e.g., an organism, etc.) by irradiation of excitation light to obtain information which is used for diagnosis, are known. These diagnosis instruments employ a method of detecting fluorescence emitted when excitation light for diagnosis is irradiated to the tissue of an organism, a method of detecting fluorescence emitted by irradiating excitation light to the tissue of an organism which has absorbed a drug for fluorescence diagnosis beforehand, or similar methods. The diagnosis instrument is incorporated into an endoscope, a colposcope, an operation microscope, etc., and is utilized for observation of a fluorescence image.
For example, Japanese Unexamined Patent Publication No. 59(1984)-40830 discloses an apparatus which employs an excimer dye laser as an excitation light source. In this apparatus, the excitation light emitted from this light source is irradiated to the tissue of an organism into which a photosensitive material having tumor affinity has been injected beforehand, and the fluorescence emitted from the tissue is observed. The above-mentioned technique is used for observing the tissue of an organism as a dynamic image by obtaining an image from the tissue at cycles of 1/60 sec and is capable of simultaneously observing a normal image and a fluorescence image as the dynamic image. For observation of the fluorescence image, the excitation light emitted from the excimer dye laser is irradiated to the tissue of an organism (which is a subject) with a pulse width of 30 nsec at cycles of 1/60 sec, and the fluorescence emitted from the tissue by irradiation of the excitation light is imaged by a high-sensitivity imaging device for a fluorescence image. In this way, the dynamic image is obtained. On the other hand, for observation of the normal image, white light is irradiated to the tissue of an organism (which is a subject) at cycles of 1/60 sec, while the aforementioned period of the irradiation of the excimer dye laser which is performed at cycles of 1/60 sec with a pulse width of 30 nsec is being avoided. The obtained images are formed into a dynamic image by an imaging device for a normal image.
Here, the pulsed light emission of an excimer dye laser will be output as pulsed light whose peak value is extremely high, even if the emission time is 30 nsec. Therefore, the intensity of fluorescence being emitted from the tissue subjected to the irradiation is sufficient to obtain satisfactory diagnosis information. In addition, there is almost no time lag between the irradiation of excitation light to the tissue and the emission of fluorescence from the tissue and therefore the irradiation of excitation light and the emission of fluorescence are considered nearly the same. Thus, there is no possibility that the period during which the irradiation of excitation light and the formation of a fluorescence image are performed will overlap with the period during which the irradiation of white light and the formation of a normal image are performed. Furthermore, because the formation of a fluorescence image is performed within the blanking period after the formation of a normal image which is a short time, the rate at which external light and background light (such as indoor illumination) are formed as noise components, along with the fluorescence image is extremely low.
As described above, while excimer dye lasers have many advantages as an excitation light source, the apparatus is extremely large in scale and extremely high in cost. Because of this, employing a small and inexpensive semiconductor laser as an excitation light source has recently been discussed.
The semiconductor laser, however, is weak in light intensity when employed as an excitation light source. In addition, if the semiconductor laser is oscillated to generate a peak value greater than or equal to the continuous maximum rated output value, a phenomenon called catastrophic optical damage (COD) will arise and the end face of the active layer of the semiconductor laser will be destroyed. In this phenomenon, non-radiative recombination occurs from a defect in the end face of the active layer of the semiconductor laser, and non-radiative recombination energy is turned into heat by the thermal vibration of the lattice. Because of this heat, the temperature of the end face rises and dislocation propagates, whereby the bandgap becomes narrower. If the bandgap becomes narrower, the end face further absorbs light and generates heat, resulting in a rise in the temperature of the end face. As a result, thermal run-away takes place and finally melts the end face. Particularly, in the semiconductor laser with a large energy gap, which is employed in an excitation light source to emit light which has a wavelength belonging to a region near ultraviolet rays, it is difficult to emit pulsed light having a peak value greater than or equal to the continuous maximum rated output value. In the case where drive current is increased to forcibly emit high-output light, degradation is conspicuous and there is a danger that a sufficiently long lifetime as a light source for a fluorescence observing apparatus will not be obtained. Also, in the case where laser light, emitted from a semiconductor laser and oscillating continuously, is used to emit high-output light which can be used as an excitation light source for a fluorescence observing apparatus, degradation is conspicuous and there is a danger that a sufficiently long lifetime will not be obtained.
The present invention has been made in view of the aforementioned problems. Accordingly, it is an object of the present invention to provide a fluorescence observing apparatus equipped with a small and inexpensive light source capable of emitting high-output excitation light. Another object of the invention is to provide a fluorescence observing apparatus which is capable of making the lifetime of the light source sufficiently long.
To achieve the aforementioned objects, there is provided a fluorescence observing apparatus comprising a light source for emitting excitation light, excitation light irradiation means for irradiating the excitation light to a sample, and fluorescence measurement means for measuring fluorescence emitted from the sample by the irradiation of the excitation light, wherein a GaN-based semiconductor laser is employed as the light source and the apparatus further includes temperature-controlling means for controlling the temperature of the semiconductor laser to 20xc2x0 C. or less.
The aforementioned objects of the present invention are also achieved by a fluorescence observing apparatus comprising a light source for emitting excitation light, excitation light irradiation means for irradiating the excitation light to a sample, and fluorescence measurement means for measuring fluorescence emitted from the sample by the irradiation of the excitation light, wherein a GaN-based semiconductor laser is employed as the light source and the apparatus further includes temperature-controlling means for controlling the temperature of the semiconductor laser to 20xc2x0 C. or less.
In a preferred form of the present invention, the aforementioned semiconductor laser emits output light greater than or equal to rated output at room temperature.
The aforementioned GaN-based semiconductor laser may be an InGaN-based semiconductor laser. In that case the active layer of the semiconductor laser may have an InGaN/InGaN quantum cell structure.
The aforementioned semiconductor laser may be a broad area type or surface emission type semiconductor laser. It may to also be an array type semiconductor laser.
According to the fluorescence observing apparatus of the present invention, in a fluorescence observing apparatus for irradiating pulsed excitation light emitted from a light source, to a sample (such as an organism, etc.) and measuring fluorescence emitted from the sample, if a GaN semiconductor laser is adopted as the light source and controlled to 20xc2x0 C. or less, the oscillating threshold current of the GaN semiconductor laser can be reduced and the maximum output is not limited at thermal saturation. As a result, the GaN semiconductor laser becomes able to oscillate with high output. In addition, since the degradation rate of the semiconductor laser becomes lower as temperature becomes lower, the semiconductor can emit high-output excitation light over a long time. Therefore, even if a small and inexpensive GaN semiconductor laser is adopted as an excitation light source for a fluorescence observing apparatus, it can emit high-output excitation light over a long time and have a sufficiently long lifetime.
If the GaN-based semiconductor layer is controlled to 10xc2x0 C. or less, it can oscillate with even higher output and further prolong its lifetime.
If the aforementioned semiconductor layer emits output light greater than or equal to rated output at room temperature, even higher output can be obtained.
If an InGaN-based semiconductor laser is employed in place of the aforementioned GaN-based semiconductor laser, carriers are inevitably captured at a local level formed due to the composition unevenness of indium (In), etc., before they are captured at a lattice defect from which non-radiative recombination occurs. At the local level, radiative recombination is performed. Therefore, even if a defect such as dislocation is present, non-radiative recombination will not occur from the defect and the injected current can be inhibited from giving rise to generation of heat without being converted to light. As a result, even if a defect such as dislocation is present within the active layer, the occurrence of catastrophic optical damage (COD) can be prevented.
If the active layer of the aforementioned semiconductor laser has an InGaN/InGaN quantum cell structure, a quantum level is formed in the quantum cell and carriers become concentrated in the mini-band. As a result, as the efficiency of radiative recombination becomes better and the oscillating threshold current is reduced, higher light output can be obtained with less drive current.
If a broad area type or surface emission type semiconductor laser is employed in place of the aforementioned semiconductor laser, a high-output excitation light source can be obtained more inexpensively.